One of the least publicized examples of impact wear is on prosthetic devices like a hip joint made from a metal cap and matching metal femur (Figure 6.6). Metal-to-metal human joint replacements are very popular in Europe (200,000/year in Switzerland in 2012) and other regions of the world because they wear much less than metal-on-plastic couples. However, when engaging in physical activities that cause disengagement of the mating members (like some types of exercise or sports activity), the metal surfaces can see impact wear and even spalling. Ceramic-on-ceramic joint prosthetics are also susceptible to impact damage. Needless to say, any spalling or material removal of any kind from these surfaces is undesirable.